The invention relates to a coating device for applying thin wet films to a carrier which is guided continuously over a support surface.
For ecological and economical reasons, development in coating technology is leading to thinner wet films at higher processing velocities. In these cases, to ensure satisfactory coating, it is necessary to place higher requirements on the freedom from disturbance of the coating process than in the case of thicker wet films.
In the case of slit-die coating, the distance of the lips from the substrate or carrier to be coated plays an important role. In general, lower wet film thicknesses or higher coating velocities require a smaller die distance, or, additionally complicated process technologies have to be used, for example in the form of a subatmospheric pressure applied to the meniscus of the wet film.
In the case of small distances, there is a high danger, as a result of particles below or on the substrate or defects in the substrate, of the slit die coming into contact therewith at high velocities and thereby of the die being damaged or the web being caused to tear. Damage to the die lip may lead to defects in the coating, so that exchange of the lips is then necessary.
To apply wet films of low thickness, according to DE-A 41 12 428 a slit die is used around which the carrier film loops. Practical difficulties arise when metal strips are to be coated since the latter, because of their high resilient restoring force, can only be guided with difficulty over the curved slit die.
In the case of this known application device of the extrusion type, the carrier film moves with a predetermined velocity and in a predetermined travel direction. The application device comprises a head which is provided with a bore running parallel to the axis of the head to receive the liquid, a slit connecting the bore to a surface of the head, a trailing-edge section which is arranged adjacent to the slit on the upstream side thereof and has a bevel whose width is less than or equal to 50 .mu.m, but larger than the diameter of any grain of the material of the trailing-edge section, and a doctor-edge section which is arranged adjacent to the slit on the downstream side thereof with respect to the travel direction, the doctor-edge section having a bevel which has a width of approximately 2 to 3 .mu.m and is provided on the upstream edge of the surface of the doctor-edge section.
One object of the invention is to carry out wet film coatings with slit dies at high velocities of the carrier and with low wet film thicknesses without the die lip being damaged or the web tearing as a result of defects in the wet film, in the carrier or by contact of the carrier with the slit die.
This object is achieved according to the invention in that a slit die of the coating device is equipped with at least one elastic lip comprising a flat strip which projects beyond a leading edge of the outlet orifice of the slit die.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. The further development of the invention ensues from the features of the claims.